Collinear array dipole antennas are well known for providing radiation over a wide range of angles around the antenna, and more particularly for providing omnidirectional radiation. Known types of collinear array antennas include the Franklin antenna, which is a series-fed collinear array typically manufactured using a coaxial cable feed line, as well as other, similar, structures. Such antennas generally include a series-fed sequence of end-fed, half wavelength radiators, which produce a substantially uniform circular radiation pattern in the azimuth.
However, most types of series-fed antenna inherently possess a narrow bandwidth. Each successive radiator is ideally separated from the source by an additional half wavelength at the designed centre frequency of the antenna. However, at frequencies different from the design frequency, the radiators are no longer separated by a half-wavelength. The resulting cumulative change in phase degrades the antenna performance at such frequencies, by causing the peak of the radiated beam to tilt up and down with increasing and decreasing frequency, thereby causing variations in radiation intensity at the horizon.
A solution to the aforementioned problem of series-fed antennas is to use a corporate, or parallel, feed arrangement, in which a dipole antenna array is fed from a common array feed point over equal length transmission paths. In a corporate feed arrangement, the phase shift from the feed point to each dipole will be substantially equal over a broad range of frequencies. The result is a more uniform radiation pattern over the bandwidth of the antenna.
One common method used to form collinear arrays of corporate-fed radiators is to side mount centre-fed dipoles off a common mast. The radiators are fed with a branched feed as previously described, to eliminate beam tilting as a function of frequency. The side mounted dipoles are typically spaced symmetrically around and close to the mast, at 90 degree increments, in order to minimize the deviation from circularity in the azimuth of each dipole. However, the cables and the mast of such antennas act as parasitic elements which reflect energy, resulting in a cardioid pattern, rather than circular pattern, of radiation emitted by each dipole. While this may be offset to some degree by the 90 degree incremental placement of the dipoles around the mast, the overall radiation pattern nonetheless deviates from circularity, and additionally the centre of the main lobe of the radiation pattern will deviate above and below the horizon to some degree, as the pattern is viewed from various sectors in the azimuth.
In an attempt to overcome the disadvantages associated with side-mounted dipoles, alternative dipole structures have been developed which can be symmetrically and collinearly mounted to produce substantially omnidirectional radiation patterns. Such antennas generally employ cylindrical or tubular radiating elements which may be mounted coaxially with a support mast to provide a uniform radiation pattern. However, precise relative placement of the cylindrical elements is essential in such antennas, since the spacing between elements of each dipole critically affects the input impedance, which in turn determines the degree of matching with the feeding transmission line and thereby the efficiency and frequency response of the antenna. The necessity to ensure accurate positioning of the individual antenna elements leads to increased complexity and cost in the design and construction of antennas of this type. In many instances, individual testing and fine tuning of an assembled antenna array is necessary to ensure that the resulting antenna meets specified bandwidth and radiation pattern requirements.
Furthermore, the large number of mechanical and electrical joints that may be required in the assembly of antennas formed from individual cylindrical elements may result in other forms of degradation in antenna performance. In particular, electrical and mechanical joints between individual metallic components of an antenna may result in a parasitic non-linear response, causing a form of degradation known as Passive Inter-Modulation distortion (PIM). In practice, PIM can result in crosstalk between signals on different RF carriers within the antenna bandwidth, and it is therefore essential to minimize this type of distortion. A typical specification for maximum acceptable PIM in a mobile radio or telephony system is −150 dBc for two carriers at 20 watts. It may be very difficult to meet this specification with an antenna having a large number of mechanical joints, in addition to which the long-term stability of antenna performance may be an issue. For example, an antenna deployed in a typical mobile telephony application will be mounted on a tower where it is subjected over time to wind, electrical hum and mechanical vibrations which may cause mechanical joints to shift or loosen, resulting in degradation of PIM performance over time.
Accordingly, there is a need for an improved collinear array dipole antenna structure that is able to provide a wide-angle radiation pattern, preferably an omnidirectional pattern, along with a broad bandwidth, while mitigating the aforementioned problems of known antennas of this type.